Conductive paste composition and semiconductor devices made therewith

ABSTRACT

A conductive paste composition contains a source of an electrically conductive metal, a boron lithium tellurium oxide, and an organic vehicle. An article such as a high-efficiency photovoltaic cell is formed by a process of deposition of the paste composition on a semiconductor device substrate (e.g., by screen printing) and firing the paste to remove the organic vehicle and sinter the metal and establish electrical contact between it and the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/911,034, filed Dec. 3, 2013 and entitled “Conductive PasteComposition and Semiconductor Devices Made Therewith,” which isincorporated herein in its entirety by reference thereto.

FIELD OF THE INVENTION

The present invention relates to a conductive paste composition that isuseful in the construction of a variety of electrical and electronicdevices, and more particularly to a paste composition useful in creatingconductive structures, including front-side electrodes for photovoltaicdevices.

TECHNICAL BACKGROUND OF THE INVENTION

A conventional photovoltaic cell incorporates a semiconductor structurewith a junction, such as a p-n junction formed with an n-typesemiconductor and a p-type semiconductor. More specifically, Si solarcells are typically made by adding controlled impurities (calleddopants) to purified Si. Different dopants result in either p-type orn-type material, in which there are respectively positive or negativemajority charge carriers. The cell structure includes a boundary orjunction between p-type and n-type Si. When the cell is illuminated byradiation of an appropriate wavelength, such as sunlight, a potential(voltage) difference across the junction creates free charge carriers.These electron-hole pair charge carriers migrate in the electric fieldgenerated by the p-n junction and are collected by electrodes onrespective surfaces of the semiconductor. The cell is thus adapted tosupply electric current to an electrical load connected to theelectrodes, thereby providing electrical energy converted from theincoming solar energy that can do useful work. For the typical p-baseconfiguration, a negative electrode is located on the side of the cellthat is to be exposed to a light source (the “front” side, which in thecase of a solar cell is the side exposed to sunlight), and a positiveelectrode is located on the other side of the cell (the “back” side).Solar-powered photovoltaic systems are considered to be environmentallybeneficial in that they reduce the need for fossil fuels used inconventional electric power plants.

Industrial photovoltaic cells are commonly provided in the form of astructure, such as one based on a doped crystalline silicon wafer, thathas been metalized, i.e., provided with electrodes in the form ofelectrically conductive metal contacts through which the generatedcurrent can flow to an external electrical circuit load. Most commonly,these electrodes are provided on opposite sides of a generally planarcell structure. Conventionally, they are produced by applying suitableconductive metal pastes to the respective surfaces of the semiconductorbody and thereafter firing the pastes.

Photovoltaic cells are commonly fabricated with an insulating layer ontheir front side to afford an anti-reflective property that maximizesthe utilization of incident light. However, in this configuration, theinsulating layer normally must be removed to allow an overlaidfront-side electrode to make contact with the underlying semiconductorsurface. The front-side conductive metal paste typically includes aglass frit and a conductive species (e.g., silver particles) carried inan organic medium that functions as a vehicle for printing. Theelectrode may be formed by depositing the paste composition in asuitable pattern (for instance, by screen printing) and thereafterfiring the paste composition and substrate to dissolve or otherwisepenetrate the insulating anti-reflective layer and sinter the metalpowder, such that an electrical connection with the semiconductorstructure is formed.

The ability of the paste composition to penetrate or etch through theanti-reflective layer and form a strong adhesive bond with the substrateupon firing is highly dependent on the composition of the conductivepaste and the firing conditions. Key measures of photovoltaic cellelectrical performance, such as efficiency, are also influenced by thequality of the electrical contact made between the fired conductivepaste and the substrate.

Although various methods and compositions useful in forming devices suchas photovoltaic cells are known, there nevertheless remains a need forcompositions that permit fabrication of patterned conductive structuresthat provide improved overall device electrical performance and thatfacilitate the efficient manufacture of such devices.

One common class of Si solar cell designs employs a 200 μm thick p-typeSi wafer with a 0.4 μm layer of n-type Si on the wafer's front surface.The p-type wafer provides the base and the n-type layer is the emitter.In various implementations the n-type layer is made by either diffusingor ion implanting phosphorus (P) dopant into the Si wafer.

Dopant concentration must be controlled to achieve optimal cellperformance. A high dopant concentration in the emitter imparts lowelectrical emitter sheet resistivity and enables a low resistivity metalcontact to be made at the Si surface, thereby decreasing resistancelosses. However, a high dopant concentration also introduces crystallinedefects or electrical perturbations in the Si lattice that increaserecombination losses that reduce both the current and voltage of thecell.

As known to one skilled in the art (see, e.g., S. W. Jones, “Diffusionin Silicon,” IC Knowledge LLC (2008), pp. 56-62), total dopantconcentration is typically measured using a SIMS (secondary ion massspectrometry) depth profiling method and active dopant concentration ismeasured using SRP (spreading resistance probing) or ECV(electrochemical capacitance voltage) methods.

The wafers most commonly used in conventional photovoltaic cells areprepared with emitters that have a total concentration of P dopant atthe surface [P_(surface)] ranging from 9 to 15×10²⁰ atoms/cm³. Theactive [P_(surface)] typically ranges from 3 to 4×10²⁰ atoms/cm³. Suchemitters are termed as highly or heavily doped emitters (HDE), and thewafers incorporating those emitters are often referenced simply as “HDEwafers.”

A concentration of P dopant at the front surface ([P_(surface)]) above˜1×10²⁰ atoms/cm³ in Si leads to various types of recombination.Recombined charge carriers are bound to the Si lattice and unable to becollected as electrical energy. The solar cell energy loss results froma decrease of both Voc (open circuit voltage) and Isc (short circuitcurrent). P dopant in excess of the active concentration (inactive P)leads to Shockley-Read-Hall (SRH) recombination energy loss. Active Pdopant above ˜1×10²⁰ atoms/cm³ leads to Auger recombination energy loss.

Emitters made with low dopant concentration at the wafer surface arecalled lightly or low-doped emitters (LDE). A wafer having[P_(surface)]<1×10²⁰ atoms/cm³ is typically termed an LDE wafer. Solarcell embodiments employing lightly doped emitters in some instancesachieve improved solar cell performance by decreasing the lossesresulting from electron-hole recombination at the front surface.However, the inherent potential of LDE-based cells to provide improvedcell performance is frequently mitigated in practice by the greaterdifficulty of forming the high-quality metal contacts needed toefficiently extract current from the operating cell.

As a result, wafers used for commercial solar cells typically employhigh [P_(surface)] emitters, as discussed above, which degrade shortwavelength response (short wavelengths have a very high absorptioncoefficient in silicon and are absorbed very close to the surface) andresult in lower open-circuit voltage (Voc) and short-circuit currentdensity (Jsc). The high [P_(surface)] emitters enable formation of lowcontact resistivity metallization contacts, without which contact ispoor and cell performance is degraded.

Nevertheless, an improvement in cell performance is potentiallyattainable with LDE-based cells. Such cells would require a thick-filmmetallization paste that can reliably contact lightly doped, low[P_(surface)] emitters without damaging the emitter layer surface, whilestill providing low contact resistance. Ideally, such a paste wouldenable screen-printed crystalline silicon solar cells to have reducedsaturation current density at the front surface (J0e) and accompanyingincreased Voc and Jsc, and therefore improved solar cell performance.Other desirable characteristics of a paste would include high bulkconductivity and the ability to form narrow, high-aspect-ratio contactlines in a metallization pattern to further reduce series resistance andminimize shading of incident light by the electrodes, as well as goodadherence to the substrate.

SUMMARY OF THE INVENTION

An embodiment of the invention relates to a paste compositioncomprising:

(a) a source of electrically conductive metal;

(b) a boron lithium tellurium oxide; and

(c) an organic vehicle in which the source of electrically conductivemetal and the boron lithium tellurium oxide are dispersed.

Embodiments of the present disclosure include ones in which the boronlithium tellurium oxide comprises 15 to 40 cation % B, 10 to 45 cation %Li, and 20 to 65 cation % Te, and ones in which the boron, lithium, andtellurium cations together comprise 80 to 100 cation % of the boronlithium tellurium oxide.

In certain embodiments, the paste composition further comprises 0.01 to5 wt. %, or 0.05 to 2.5 wt. %, or 0.1 to 1 wt. % of a discrete oxideadditive, or a metal or compound that generates an oxide upon firing.

Another aspect provides a process for forming an electrically conductivestructure on a substrate, the process comprising:

-   -   (a) providing a substrate having a first major surface and an        insulating layer thereon that comprises aluminum oxide, titanium        oxide, silicon nitride, SiN_(x):H, silicon oxide, or silicon        oxide/titanium oxide;    -   (b) applying a paste composition onto a preselected portion of        the insulating layer on the first major surface, wherein the        paste composition comprises:        -   i) a source of electrically conductive metal,        -   ii) a boron lithium tellurium oxide, and        -   iii) an organic vehicle in which the source of electrically            conductive metal and the boron lithium tellurium oxide are            dispersed; and    -   (c) firing the substrate and paste composition thereon, wherein        the insulating layer is penetrated and the electrically        conductive metal is sintered during the firing to form the        electrically conductive structure and provide electrical contact        between the electrically conductive metal and the substrate.

In a further implementation, the substrate includes an anti-reflectivelayer on its surface, and the firing results in the paste at leastpartially etching through the anti-reflective layer, such thatelectrical contact between the conductive structure and the substrate isestablished.

Further, there is provided an article comprising a substrate and anelectrically conductive structure thereon, the article having beenformed by the foregoing process. Representative articles of this typeinclude a semiconductor device and a photovoltaic cell. In anembodiment, the substrate comprises a silicon wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the preferred embodiments of the invention and theaccompanying drawings, wherein like reference numerals denote similarelements throughout the several views and in which:

FIGS. 1A-1F illustrate successive steps of a process by which asemiconductor device may be fabricated. The device in turn may beincorporated into a photovoltaic cell.

Reference numerals as used in FIGS. 1A-1F include the following:

-   -   10: p-type substrate    -   12: first major surface (front side) of substrate 10    -   14: second major surface (back side) of substrate 10    -   20: n-type diffusion layer    -   30: insulating layer    -   40: p+ layer    -   60: aluminum paste formed on back side    -   61: aluminum back electrode (obtained by firing back-side        aluminum paste)    -   70: silver or silver/aluminum paste formed on back side    -   71: silver or silver/aluminum back electrode (obtained by firing        back-side paste)    -   500: conductive paste formed on front side according to the        invention    -   501: conductive front electrode according to the invention        (formed by firing front-side conductive paste)

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses the need for a process to manufacturehigh performance semiconductor devices having mechanically robust, highconductivity electrodes. The conductive paste composition providedherein is beneficially employed in the fabrication of front-sideelectrodes of photovoltaic devices. Ideally, a paste compositionpromotes the formation of a front-side metallization that: (a) adheresstrongly to the underlying semiconductor substrate; and (b) provides arelatively low resistance contact with the substrate. Suitable pastecompositions are believed to aid in etching surface insulating layersoften employed in semiconductor structures such as photovoltaic cells toallow contact between the conductive electrode and the underlyingsemiconductor.

In an aspect, this invention provides a paste composition thatcomprises: a functional conductive component, such as a source ofelectrically conductive metal; a boron lithium tellurium oxide; anoptional discrete inorganic additive; and an organic vehicle. Certainembodiments involve a photovoltaic cell that includes a conductivestructure made with the present paste composition. Such cells mayprovide any combination of one or more of high photovoltaic conversionefficiency, high fill factor, and low series resistance.

In various embodiments, the present paste composition may comprise aninorganic solids portion comprising (a) about 75% to about 99.5% byweight, or about 90% to about 99% by weight, or about 95% to about 99%by weight, of a source of an electrically conductive metal; (b) about 1%to about 15% by weight, or about 1% to about 8% by weight, or about 2%to about 6% by weight, or about 1% to about 5% by weight, or about 1% toabout 3% by weight, of a boron lithium tellurium oxide material, whereinthe above stated contents of constituents (a) and (b) are based on thetotal weight of all the constituents of the inorganic solids portion ofthe composition, apart from the organic medium.

As further described below, the paste composition further comprises anorganic vehicle, which acts as a carrier for the inorganic constituents,which are dispersed therein. The paste composition may include stilladditional components such as surfactants, thickeners, thixotropes, andbinders.

Typically, electrodes and other conductive traces are provided by screenprinting the paste composition onto a substrate, although other forms ofprinting, such as plating, extrusion, inkjet, shaped or multipleprinting, or ribbons may also be used. After deposition, thecomposition, which typically comprises a conductive metal powder (e.g.,Ag), a frit, and optional inorganic additives in an organic carrier, isfired at an elevated temperature.

The composition also can be used to form conductive traces, such asthose employed in a semiconductor module that is to be incorporated intoan electrical or electronic device. As would be recognized by a skilledartisan, the paste composition described herein can be termed“conductive,” meaning that the composition can be formed into astructure and thereafter processed to exhibit an electrical conductivitysufficient for conducting electrical current between devices orcircuitry connected thereto.

I. Inorganic Components

An embodiment of the present invention relates to a paste composition,which may include: an inorganic solids portion comprising a functionalmaterial providing electrical conductivity, a boron lithium telluriumoxide fusible material; and an organic vehicle in which the inorganicsolids are dispersed. The paste composition may further includeadditional components such as surfactants, thickeners, thixotropes, andbinders.

A. Electrically Conductive Metal

The present paste composition includes a source of an electricallyconductive metal. Exemplary metals include without limitation silver,gold, copper, nickel, palladium, platinum, aluminum, and alloys andmixtures thereof. Silver is preferred for its processability and highconductivity. However, a composition including at least somenon-precious metal may be used to reduce cost.

The conductive metal may be incorporated directly in the present pastecomposition as a metal powder. In another embodiment, a mixture of twoor more such metals is directly incorporated. Alternatively, the metalis supplied by a metal oxide or salt that decomposes upon exposure tothe heat of firing to form the metal. As used herein, the term “silver”is to be understood as referring to elemental silver metal, alloys ofsilver, and mixtures thereof, and may further include silver derivedfrom silver oxide (Ag₂O or AgO) or silver salts such as AgCl, AgNO₃,AgOOCCH₃ (silver acetate), AgOOCF₃ (silver trifluoroacetate), Ag₃PO₄(silver orthophosphate), or mixtures thereof. Any other form ofconductive metal compatible with the other components of the pastecomposition also may be used.

Electrically conductive metal powder used in the present pastecomposition may be supplied as finely divided particles having any oneor more of the following morphologies: a powder form, a flake form, aspherical form, a rod form, a granular form, a nodular form, acrystalline form, other irregular forms, or mixtures thereof. Theelectrically conductive metal or source thereof may also be provided ina colloidal suspension, in which case the colloidal carrier would not beincluded in any calculation of weight percentages of the solids of whichthe colloidal material is part.

The particle size of the metal is not subject to any particularlimitation. As used herein, “particle size” is intended to refer to“median particle size” or d₅₀, by which is meant the 50% volumedistribution size. The distribution may also be characterized by d₉₀,meaning that 90% by volume of the particles are smaller than d₉₀. Volumedistribution size may be determined by a number of methods understood byone of skill in the art, including but not limited to laser diffractionand dispersion methods employed by a Microtrac particle size analyzer(Montgomeryville, Pa.). Laser light scattering, e.g., using a modelLA-910 particle size analyzer available commercially from HoribaInstruments Inc. (Irvine, Calif.), may also be used. In variousembodiments, the median particle size is greater than 0.2 μm and lessthan 10 μm, or the median particle size is greater than 0.4 μm and lessthan 5 μm, as measured using the Horiba LA-910 analyzer.

The electrically conductive metal may comprise any of a variety ofpercentages of the composition of the paste composition. To attain highconductivity in a finished conductive structure, it is generallypreferable to have the concentration of the electrically conductivemetal be as high as possible while maintaining other requiredcharacteristics of the paste composition that relate to eitherprocessing or final use. In an embodiment, the silver or otherelectrically conductive metal may comprise about 75% to about 99% byweight, or about 85% to about 99% by weight, or about 95% to about 99%by weight, of the inorganic solid components of the paste composition.In another embodiment, the solids portion of the paste composition mayinclude about 80 wt. % to about 90 wt. % silver particles and about 1wt. % to about 9 wt. % silver flakes. In an embodiment, the solidsportion of the paste composition may include about 70 wt. % to about 90wt. % silver particles and about 1 wt. % to about 9 wt. % silver flakes.In another embodiment, the solids portion of the paste composition mayinclude about 70 wt. % to about 90 wt. % silver flakes and about 1 wt. %to about 9 wt. % of colloidal silver. In a further embodiment, thesolids portion of the paste composition may include about 60 wt. % toabout 90 wt. % of silver particles or silver flakes and about 0.1 wt. %to about 20 wt. % of colloidal silver.

The electrically conductive metal used herein, particularly when inpowder form, may be coated or uncoated; for example, it may be at leastpartially coated with a surfactant to facilitate processing. Suitablecoating surfactants include, for example, stearic acid, palmitic acid, asalt of stearate, a salt of palmitate, and mixtures thereof. Othersurfactants that also may be utilized include lauric acid, oleic acid,capric acid, myristic acid, linoleic acid, and mixtures thereof. Stillother surfactants that also may be utilized include polyethylene oxide,polyethylene glycol, benzotriazole, poly(ethylene glycol)acetic acid,and other similar organic molecules. Suitable counter-ions for use in acoating surfactant include without limitation hydrogen, ammonium,sodium, potassium, and mixtures thereof. When the electricallyconductive metal is silver, it may be coated, for example, with aphosphorus-containing compound.

In an embodiment, one or more surfactants may be included in the organicvehicle in addition to any surfactant included as a coating ofconductive metal powder used in the present paste composition.

As further described below, the electrically conductive metal can bedispersed in an organic vehicle that acts as a carrier for the metalphase and other constituents present in the formulation.

B. Boron Lithium Tellurium Oxide

The present paste composition includes a fusible boron lithium telluriumoxide. The term “fusible,” as used herein, refers to the ability of amaterial to become fluid upon heating, such as the heating employed in afiring operation. In some embodiments, the fusible material is composedof one or more fusible subcomponents. For example, the fusible materialmay comprise a glass material, or a mixture of two or more glassmaterials. Glass material in the form of a fine powder, e.g., as theresult of a comminution operation, is often termed “frit” and is readilyincorporated in the present paste composition.

As used herein, the term “glass” refers to a particulate solid form,such as an oxide or oxyfluoride, that is at least predominantlyamorphous, meaning that short-range atomic order is preserved in theimmediate vicinity of any selected atom, that is, in the firstcoordination shell, but dissipates at greater atomic-level distances(i.e., there is no long-range periodic order). Hence, the X-raydiffraction pattern of a fully amorphous material exhibits broad,diffuse peaks, and not the well-defined, narrow peaks of a crystallinematerial. In the latter, the regular spacing of characteristiccrystallographic planes give rise to the narrow peaks, whose position inreciprocal space is in accordance with Bragg's law. A glass materialalso does not show a substantial crystallization exotherm upon heatingclose to or above its glass transition temperature or softening point,T_(g), which is defined as the second transition point seen in adifferential thermal analysis (DTA) scan. In an embodiment, thesoftening point of glass material used in the present paste compositionis in the range of 300 to 800° C.

It is also contemplated that some or all of the boron lithium telluriumoxide material may be composed of material that exhibits some degree ofcrystallinity. For example, in some embodiments, a plurality of oxidesare melted together, resulting in a material that is partially amorphousand partially crystalline. As would be recognized by a skilled person,such a material would produce an X-ray diffraction pattern havingnarrow, crystalline peaks superimposed on a pattern with broad, diffusepeaks. Alternatively, one or more constituents, or even substantiallyall of the fusible material, may be predominantly or even substantiallyfully crystalline. In an embodiment, crystalline material useful in thefusible material of the present paste composition may have a meltingpoint of at most 800° C.

The fusible material used in the present paste composition is a boronlithium tellurium oxide. As used herein, the term “boron lithiumtellurium oxide” refers to an oxide material containing boron, lithium,and tellurium cations that together comprise at least 80% of the cationspresent in the material, and wherein the minimum content of boron,lithium, and tellurium cations is at least 15, 10, and 20 cation %,respectively. In various embodiments, the combination of boron, lithium,and tellurium cations represents at least 80%, 90%, 95%, or up to 100%of the cations in the boron lithium tellurium oxide.

The boron lithium tellurium oxide used in the present paste compositionis described herein as including percentages of certain components.Specifically, the composition may be specified by denominatingindividual components that may be combined in the specified percentagesto form a starting material that subsequently is processed, e.g., asdescribed herein, to form a glass or other fusible material. Suchnomenclature is conventional to one of skill in the art. In other words,the composition contains certain components, and the percentages ofthose components may be expressed as weight percentages of thecorresponding oxide or other forms.

Alternatively, some of the compositions herein are set forth by cationpercentages, which are based on the total cations contained in the boronlithium tellurium oxide. Of course, compositions thus specified includethe oxygen or other anions associated with the various cations. Askilled person would recognize that compositions could equivalently bespecified by weight percentages of the constituents, and would be ableto perform the required numerical conversions.

The boron lithium tellurium oxide included in the present pastecomposition optionally incorporates other oxides, including oxides ofone or more of the elements Al, Na, K, Rb, Cs, Ti, V, Cr, Mn, Fe, Co,Ni, Cu, Zn, Zr, Nb, Si, Mo, Hf, W, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, Bi,P, Y, La and the other lanthanide elements, and mixtures thereof. (Theterm “lanthanide elements” is understood to include the elements of theperiodic table having atomic numbers of 57 through 71, i.e., La—Lu.)This list is meant to be illustrative, not limiting. In variousembodiments, the boron lithium tellurium oxide used in present pastecomposition may incorporate an oxide of the foregoing cations, or anymixture thereof, at up to a total of 5 cation %, or 10 cation %, or 15cation %, or 20 cation %. Silver oxide may be included at up to 10cation %, or 15 cation %, or 20 cation %. Such substances are intimatelymixed at an atomic level in the boron lithium tellurium oxide, e.g., bymelting the substances together. In different embodiments, the amount ofthese other oxides incorporated is such that the total cation percentageof them in the boron lithium tellurium oxide is up to 20%.

Although oxygen is typically the predominant anion in the boron lithiumtellurium oxide of the present paste composition, some portion of theoxygen may be replaced by fluorine or other halide anions to altercertain properties, such as chemical, thermal, or rheological propertiesof the oxide that affect firing. In an embodiment, up to 10% of theoxygen anions of the boron lithium tellurium oxide in any of theformulations of the present paste composition are replaced by one ormore halogen anions, including fluorine. For example, up to 10% of theoxygen anions may be replaced by fluorine. Halide anions may be suppliedfrom halides of any of the composition's cations, including, but notlimited to, NaCl, KBr, NaI, LiF, CaF₂, MgF₂, BaCl₂, and BiF₃.

For example, one of ordinary skill would recognize that embodimentswherein the boron lithium tellurium oxide contains fluorine can beprepared using fluorine anions supplied from a simple fluoride or anoxyfluoride. In an embodiment, the desired fluorine content can besupplied by replacing some or all of an oxide nominally incorporated inthe composition with the corresponding fluoride of the same cation, suchas by replacing some or all of the Li₂O nominally included with theamount of LiF needed to attain the desired level of F content. Ofcourse, the requisite amount of F can be derived by replacing the oxidesof more than one cation of the boron lithium tellurium oxide if desired.Other fluoride sources could also be used, including sources such asammonium fluoride that would decompose during the heating in typicalglass preparation to leave behind residual fluoride anions. Usefulfluorides include, but are not limited to, CaF₂, BiF₃, AlF₃, NaF, LiF,ZrF₄, TiF₄, and ZnF₂.

The present paste composition may further comprise an optional discreteoxide additive. It is contemplated that the additive may comprise anoxide of one element, two or more discrete oxides of various elements,or a discrete mixed oxide of multiple elements. As used herein, the term“oxide of an element” includes both the oxide compound itself and anyother organic or inorganic compound of the element, or the pure elementitself if it oxidizes or decomposes on heating to form the pertinentoxide. Such compounds known to decompose upon heating include, but arenot limited to, carbonates, nitrates, nitrites, hydroxides, acetates,formates, citrates, and soaps of the foregoing elements, and mixturesthereof. For example, Zn metal, zinc acetate, zinc carbonate, and zincmethoxide are potential additives that would oxidize or decompose toform zinc oxide upon firing. The oxide is discrete, in that it is notmixed at an atomic level with the base boron lithium tellurium oxidefrit, but is separately present in the paste composition. In anembodiment, the discrete oxide additive may be present in the pastecomposition in an amount ranging from 0.01 to 5 wt. %, or 0.05 to 2.5wt. %, or 0.1 to 1 wt. %, based on the total weight of the pastecomposition.

Although in some embodiments the present composition (including thefusible material contained therein) may contain a small amount of lead,lead oxide, or other lead compound, e.g., in an amount up to 5 cation %in the boron lithium tellurium oxide, other embodiments are lead-free.As used herein, the term “lead-free paste composition” refers to a pastecomposition to which no lead has been specifically added (either aselemental lead or as a lead-containing alloy, compound, or other likesubstance), and in which the amount of lead present as a trace componentor impurity is 1000 parts per million (ppm) or less. In someembodiments, the amount of lead present as a trace component or impurityis less than 500 ppm, or less than 300 ppm, or less than 100 ppm.Surprisingly and unexpectedly, photovoltaic cells exhibiting desirableelectrical properties, such as high conversion efficiency, are obtainedin some embodiments of the present disclosure, notwithstanding previousbelief in the art that substantial amounts of lead must be included in apaste composition to attain these levels.

Similarly, embodiments of the present paste composition comprisecadmium, e.g., in an amount up to 5 cation % in the boron lithiumtellurium oxide, while others are cadmium-free, again meaning that no Cdmetal or compound is specifically added and that the amount present as atrace impurity is less than 1000 ppm, 500 ppm, 300 ppm, or 100 ppm.

In other embodiments, the boron lithium tellurium oxide of the presentpaste composition comprises, or consists essentially of:

15 to 40, or 20 to 40, or 25 to 35 cation % of B;

10 to 45, or 10 to 35, or 10 to 17.5, or 15 to 35 cation % of Li; and

20 to 65, or 25 to 55, or 20 to 29.5, or 20 to 24.5, or 35 to 55 cation% of Te, plus incidental impurities.

In still other embodiments, the boron lithium tellurium oxide of thepresent paste composition comprises, or consists essentially of:

15 to 40, or 20 to 40, or 25 to 35 cation % of B;

10 to 45, or 10 to 35, or 10 to 17.5, or 15 to 35 cation % of Li;

20 to 65, or 25 to 55, or 20 to 29.5, or 20 to 24.5, or 35 to 55 cation% of Te; and

one or both of 0 to 2, 0 to 1.5, or 0.5 to 1.5 cation % of Ti; and 0 to4.5 cation % of one or more of the alkaline earth metals Mg, Ca, Sr, andBa,

plus incidental impurities.

For example, the boron lithium tellurium oxide of the present pastecomposition may comprise, or consist essentially of:

15 to 40, or 20 to 40, or 25 to 35 cation % of B;

10 to 45, or 10 to 35, or 10 to 17.5, or 15 to 35 cation % of Li;

20 to 29.5 cation % of Te; and

0 to 4.5 cation % of one or more of the alkaline earth metals Mg, Ca,Sr, and Ba,

plus incidental impurities.

One of ordinary skill in the art of glass chemistry would furtherrecognize that any of the foregoing boron lithium tellurium oxidematerial compositions, whether specified by weight percentages or cationpercentages of its constituent oxides, may alternatively be prepared bysupplying the required anions and cations in requisite amounts fromdifferent components that, when mixed and fired, yield the same overallcomposition. For example, in various embodiments, phosphorus could besupplied either from P₂O₅, or alternatively from a suitable organic orinorganic phosphate that decomposes on heating to yield P₂O₅, or from ametal phosphate in which the metal is also a desired component of thefinal material. The skilled person would also recognize that a certainportion of volatile species, e.g., carbon dioxide, may be releasedduring the process of making a fusible material.

It is known to those skilled in the art that a boron lithium telluriumoxide such as one prepared by a melting technique as described hereinmay be characterized by known analytical methods that include, but arenot limited to: Inductively Coupled Plasma-Emission Spectroscopy(ICP-ES), Inductively Coupled Plasma-Atomic Emission Spectroscopy(ICP-AES), and the like. In addition, the following exemplary techniquesmay be used: X-Ray Fluorescence spectroscopy (XRF), Nuclear MagneticResonance spectroscopy (NMR), Electron Paramagnetic Resonancespectroscopy (EPR), Mössbauer spectroscopy, electron microprobe EnergyDispersive Spectroscopy (EDS), electron microprobe Wavelength DispersiveSpectroscopy (WDS), and Cathodoluminescence (CL). A skilled person couldcalculate percentages of starting components that could be processed toyield a particular fusible material, based on results obtained with suchanalytical methods.

The embodiments of the boron lithium tellurium oxide material describedherein, including the compositions listed in Tables I and IV are notlimiting; it is contemplated that one of ordinary skill in the art ofglass chemistry could make minor substitutions of additional ingredientsand not substantially change the desired properties of the boron lithiumtellurium oxide composition, including its interaction with a substrateand any insulating layer thereon.

A median particle size of the boron lithium tellurium oxide material inthe present composition may be in the range of about 0.5 to 10 μm, orabout 0.8 to 5 μm, or about 1 to 3 μm, as measured using the HoribaLA-910 analyzer.

In an embodiment, the boron lithium tellurium oxide may be produced byconventional glass-making techniques and equipment. For the examplesprovided herein, the ingredients were weighed and mixed in the desiredproportions and heated in a platinum alloy crucible in a furnace. Theingredients may be heated to a peak temperature (e.g., 800° C. to 1400°C., or 900° C. to 1050° C.) and held for a time such that the materialforms a melt that is substantially liquid and homogeneous (e.g., 20minutes to 2 hours). The melt optionally is stirred, eitherintermittently or continuously. In an embodiment, the melting processresults in a material wherein the constituent chemical elements arefully mixed at an atomic level. The molten material is then typicallyquenched in any suitable way including, without limitation, passing itbetween counter-rotating stainless steel rollers to form 0.25 to 0.50 mmthick platelets, by pouring it onto a thick stainless steel plate, or bypouring it into water or other quench fluid. The resulting particles arethen milled to form a powder or frit, which typically may have a d₅₀ of0.2 to 3.0 μm.

Other production techniques may also be used for the present boronlithium tellurium oxide material. One skilled in the art of producingsuch materials might therefore employ alternative synthesis techniquesincluding, but not limited to, melting in non-precious metal crucibles,melting in ceramic crucibles, sol-gel, spray pyrolysis, or othersappropriate for making powder forms of glass.

A skilled person would recognize that the choice of raw materials couldunintentionally include impurities that may be incorporated into theboron lithium tellurium oxide material during processing. For example,these incidental impurities may be present in the range of hundreds tothousands of parts per million. Impurities commonly occurring inindustrial materials used herein are known to one of ordinary skill.

The presence of the impurities would not substantially alter theproperties of the boron lithium tellurium oxide itself, pastecompositions made with the boron lithium tellurium oxide, or a fireddevice manufactured using the paste composition. For example, a solarcell employing a conductive structure made using the present pastecomposition may have the efficiency described herein, even if thecomposition includes impurities.

The boron lithium tellurium oxide used in the present composition isbelieved to assist in the partial or complete penetration of the oxideand/or nitride insulating layer commonly present on a siliconsemiconductor wafer during firing. As described herein, this at leastpartial penetration may facilitate the formation of an effective,mechanically robust electrical contact between a conductive structuremanufactured using the present composition and the underlying siliconsemiconductor of a photovoltaic device structure.

The boron lithium tellurium oxide material in the present pastecomposition may optionally comprise a plurality of separate fusiblesubstances, such as one or more frits, or a substantially crystallinematerial with additional frit material. In an embodiment, a firstfusible subcomponent is chosen for its capability to rapidly etch aninsulating layer, such as that typically present on the front surface ofa photovoltaic cell; further, the first fusible subcomponent may havestrong etching power and low viscosity. A second fusible subcomponent isoptionally included to slowly blend with the first fusible subcomponentto alter the chemical activity. Preferably, the composition is such thatthe insulating layer is partially removed but without attacking theunderlying emitter diffused region, which would shunt the device, werethe corrosive action to proceed unchecked. Such fusible materials may becharacterized as having a viscosity sufficiently high to provide astable manufacturing window to remove insulating layers without damageto the diffused p-n junction region of a semiconductor substrate.Ideally, the firing process results in a substantially complete removalof the insulating layer without further combination with the underlyingSi substrate or the formation of substantial amounts of non-conductingor poorly conducting inclusions.

C. Optional Oxide Additive

As noted above, an optional oxide may be included in the present pastecomposition as a discrete additive, such as an oxide of one or more ofAl, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn,Zr, Nb, Si, Mo, Hf, W, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, Bi, P, Y, La, ormixtures thereof, or a substance that forms such an oxide upon heating.The oxide additive can be incorporated in the paste composition in apowder form as received from the supplier, or the powder can be groundor milled to a smaller average particle size. Particles of any size canbe employed, as long as they can be incorporated into the present pastecomposition and provide its required functionality. In an embodiment,the paste composition comprises up to 5 wt. % of the discrete oxideadditive.

Any size-reduction method known to those skilled in the art can beemployed to reduce particle size to a desired level. Such processesinclude, without limitation, ball milling, media milling, jet milling,vibratory milling, and the like, with or without a solvent present. If asolvent is used, water is the preferred solvent, but other solvents maybe employed as well, such as alcohols, ketones, and aromatics.Surfactants may be added to the solvent to aid in the dispersion of theparticles, if desired.

II. Organic Vehicle

The inorganic components of the present composition are typically mixedwith an organic vehicle to form a relatively viscous material referredto as a “paste” or an “ink” that has a consistency and rheology thatrender it suitable for printing processes, including without limitationscreen printing. The mixing is typically done with a mechanical system,and the constituents may be combined in any order, as long as they areuniformly dispersed and the final formulation has characteristics suchthat it can be successfully applied during end use.

A wide variety of inert materials can be admixed in an organic medium inthe present composition including, without limitation, an inert,non-aqueous liquid that may or may not contain thickeners, binders, orstabilizers. By “inert” is meant a material that may be removed by afiring operation without leaving any substantial residue and that has noother effects detrimental to the paste or the final conductor lineproperties.

The proportions of organic vehicle and inorganic components in thepresent paste composition can vary in accordance with the method ofapplying the paste and the kind of organic vehicle used. In anembodiment, the present paste composition typically contains about 50 to95 wt. %, 76 to 95 wt. %, or 85 to 95 wt. %, of the inorganic componentsand about 5 to 50 wt. %, 5 to 24 wt. %, or 5 to 15 wt. %, of the organicvehicle.

The organic vehicle typically provides a medium in which the inorganiccomponents are dispersible with a good degree of stability. Inparticular, the composition preferably has a stability compatible notonly with the requisite manufacturing, shipping, and storage, but alsowith conditions encountered during deposition, e.g., by a screenprinting process. Ideally, the rheological properties of the vehicle aresuch that it lends good application properties to the composition,including stable and uniform dispersion of solids, appropriate viscosityand thixotropy for printing, appropriate wettability of the paste solidsand the substrate on which printing will occur, a rapid drying rateafter deposition, and stable firing properties.

Substances useful in the formulation of the organic vehicle of thepresent paste composition include, without limitation, ones disclosed inU.S. Pat. No. 7,494,607 and International Patent Application PublicationNo. WO 2010/123967 A2, both of which are incorporated herein in theirentirety for all purposes, by reference thereto. The disclosedsubstances include ethylhydroxyethyl cellulose, wood rosin, mixtures ofethyl cellulose and phenolic resins, cellulose acetate, celluloseacetate butyrate, polymethacrylates of lower alcohols, monobutyl etherof ethylene glycol, monoacetate ester alcohols, and terpenes such asalpha- or beta-terpineol or mixtures thereof with other solvents such askerosene, dibutylphthalate, butyl carbitol, butyl carbitol acetate,hexylene glycol, and high-boiling alcohols and alcohol esters.

Solvents useful in the organic vehicle include, without limitation,ester alcohols and terpenes such as alpha- or beta-terpineol or mixturesthereof with other solvents such as kerosene, dibutylphthalate, butylcarbitol, butyl carbitol acetate, hexylene glycol, and high-boilingalcohols and alcohol esters. A preferred ester alcohol is themonoisobutyrate of 2,2,4-trimethyl-1,3-pentanediol, which is availablecommercially from Eastman Chemical (Kingsport, Tenn.) as TEXANOL™. Someembodiments may also incorporate volatile liquids in the organic vehicleto promote rapid hardening after application on the substrate. Variouscombinations of these and other solvents are formulated to provide thedesired viscosity and volatility.

In an embodiment, the organic vehicle may include one or more componentsselected from the group consisting of: bis(2-(2butoxyethoxy)ethyl)adipate, dibasic esters, octyl epoxy tallate, isotetradecanol, and apentaerythritol ester of hydrogenated rosin. The paste compositions mayalso include additional additives or components.

The dibasic ester useful in the present paste composition may compriseone or more dimethyl esters selected from the group consisting ofdimethyl ester of adipic acid, dimethyl ester of glutaric acid, anddimethyl ester of succinic acid. Various forms of such materialscontaining different proportions of the dimethyl esters are availableunder the DBE® trade name from Invista (Wilmington, Del.). For thepresent paste composition, a preferred version is sold as DBE-3 and issaid by the manufacturer to contain 85 to 95 weight percent dimethyladipate, 5 to 15 weight percent dimethyl glutarate, and 0 to 1.0 weightpercent dimethyl succinate based on total weight of dibasic ester.

Further ingredients optionally may be incorporated in the organicvehicle, such as thickeners, stabilizers, and/or other common additivesknown to those skilled in the art. The organic vehicle may be a solutionof one or more polymers in a solvent. Additionally, effective amounts ofadditives, such as surfactants or wetting agents, may be a part of theorganic vehicle. Such added surfactant may be included in the organicvehicle in addition to any surfactant included as a coating on theconductive metal powder of the paste composition. Suitable wettingagents include phosphate esters and soya lecithin. Both inorganic andorganic thixotropes may also be present.

Among the commonly used organic thixotropic agents are hydrogenatedcastor oil and derivatives thereof, but other suitable agents may beused instead of, or in addition to, these substances. It is, of course,not always necessary to incorporate a thixotropic agent since thesolvent and resin properties coupled with the shear thinning inherent inany suspension may alone be suitable in this regard.

A polymer frequently used in printable conductive metal pastes is ethylcellulose. Other exemplary polymers that may be used includeethylhydroxyethyl cellulose, wood rosin and derivatives thereof,mixtures of ethyl cellulose and phenolic resins, cellulose acetate,cellulose acetate butyrate, poly(methacrylate)s of lower alcohols, andmonoalkyl ethers of ethylene glycol monoacetate.

Any of these polymers may be dissolved in a suitable solvent, includingthose described herein.

The polymer in the organic vehicle may be present in the range of 0.1wt. % to 5 wt. % of the total composition. The present paste compositionmay be adjusted to a predetermined, screen-printable viscosity, e.g.,with additional solvent(s).

III. Formation of Conductive Structures

An aspect of the invention provides a process that may be used to form aconductive structure on a substrate. The process generally comprises thesteps of providing the substrate, applying a paste composition, andfiring the substrate. Ordinarily, the substrate is planar and relativelythin, thus defining first and second major surfaces on its oppositesides.

Application

The present composition can be applied as a paste onto a preselectedportion of a major surface of the substrate in a variety of differentconfigurations or patterns. The preselected portion may comprise anyfraction of the total first major surface area, including substantiallyall of the area. In an embodiment, the paste is applied on asemiconductor substrate, which may be single-crystal, cast mono,multi-crystal, polycrystalline, or ribbon silicon, or any othersemiconductor material.

The application can be accomplished by a variety of depositionprocesses, including printing. Exemplary deposition processes include,without limitation, plating, extrusion or co-extrusion, dispensing froma syringe, and screen, inkjet, shaped, multiple, and ribbon printing.The paste composition ordinarily is applied over any insulating layerpresent on the first major surface of the substrate.

The conductive composition may be printed in any useful pattern. Forexample, the electrode pattern used for the front side of a photovoltaiccell commonly includes a plurality of narrow grid lines or fingersconnected to one or more bus bars. In an embodiment, the width of thelines of the conductive fingers may be 20 to 200 μm; 25 to 100 μm; or 35to 75 μm. In an embodiment, the thickness of the lines of the conductivefingers may be 5 to 50 μm; 10 to 35 μm; or 15 to 30 μm. Such a patternpermits the generated current to be extracted without undue resistiveloss, while minimizing the area of the front side obscured by themetallization, which reduces the amount of incoming light energy thatcan be converted to electrical energy. Ideally, the features of theelectrode pattern should be well defined, with a preselected thicknessand shape, and have high electrical conductivity and low contactresistance with the underlying structure.

Conductors formed by printing and firing a paste such as that providedherein are often denominated as “thick-film” conductors, since they areordinarily substantially thicker than traces formed by atomisticprocesses, such as those used in fabricating integrated circuits. Forexample, thick-film conductors may have a thickness after firing ofabout 1 to 100 μm. Consequently, paste compositions that in theirprocessed form provide conductivity and are suitably applied usingprinting processes are often called “thick-film pastes” or “conductiveinks.”

Firing

A firing operation may be used in the present process to effect asubstantially complete burnout of the organic vehicle from the depositedpaste. The firing typically involves volatilization and/or pyrolysis ofthe organic materials. A drying operation optionally precedes the firingoperation, and is carried out at a modest temperature to harden thepaste composition by removing its most volatile organics.

The firing process is believed to remove the organic vehicle, sinter theconductive metal in the composition, and establish electrical contactbetween the semiconductor substrate and the fired conductive metal.Firing may be performed in an atmosphere composed of air, nitrogen, aninert gas, or an oxygen-containing mixture such as a mixed gas of oxygenand nitrogen.

In one embodiment, the temperature for the firing may be in the rangebetween about 300° C. to about 1000° C., or about 300° C. to about 525°C., or about 300° C. to about 650° C., or about 650° C. to about 1000°C. The firing may be conducted using any suitable heat source. In anembodiment, the firing is accomplished by passing the substrate bearingthe printed paste composition pattern through a belt furnace at hightransport rates, for example between about 100 to about 500 cm perminute, with resulting hold-up times between about 0.05 to about 5minutes. Multiple temperature zones may be used to control the desiredthermal profile, and the number of zones may vary, for example, between3 to 11 zones. The temperature of a firing operation conducted using abelt furnace is conventionally specified by the furnace set point in thehottest zone of the furnace, but it is known that the peak temperatureattained by the passing substrate in such a process is somewhat lowerthan the highest set point. Other batch and continuous rapid firefurnace designs known to one of skill in the art are also contemplated.

In a further embodiment, other conductive and device enhancing materialsare applied prior to firing to the opposite type region of thesemiconductor device. The various materials may be applied and thenco-fired, or they may be applied and fired sequentially.

In an embodiment, the opposite type region may be on the non-illuminated(back) side of the device, i.e., its second major surface. The materialsserve as electrical contacts, passivating layers, and solderable tabbingareas. In an aspect of this embodiment, the back-side conductivematerial may contain aluminum. Exemplary back-side aluminum-containingcompositions and methods of application are described, for example, inUS 2006/0272700, which is hereby incorporated herein in its entirety forall purposes by reference thereto. Suitable solderable tabbing materialsinclude those containing silver, silver and aluminum, and other mixturesof silver and base metals. Exemplary tabbing compositions containingaluminum and silver are described, for example in US 2006/0231803, whichis hereby incorporated herein in its entirety for all purposes byreference thereto.

In a further embodiment, the present paste composition may be employedin the construction of semiconductor devices wherein the p and n regionsare formed side-by-side in a substrate, instead of being respectivelyadjacent to opposite major surfaces of the substrate. In animplementation in this configuration, the electrode-forming materialsmay be applied in different portions of a single side of the substrate,e.g., on the non-illuminated (back) side of the device, therebymaximizing the amount of light incident on the illuminated (front) side.

Insulating Layer

In some embodiments of the invention, the paste composition is used inconjunction with a substrate, such as a semiconductor substrate, havingan insulating layer present on one or more of the substrate's majorsurfaces. The layer may comprise one or more components selected fromaluminum oxide, titanium oxide, silicon nitride, SiN_(x):H (siliconnitride containing hydrogen for passivation during subsequent firingprocessing), silicon oxide, and silicon oxide/titanium oxide, and may bein the form of a single, homogeneous layer or multiple sequentialsub-layers of any of these materials. Silicon nitride and SiN_(x):H arewidely used.

The insulating layer provides some embodiments of the cell with ananti-reflective property, which lowers the cell's surface reflectance oflight incident thereon, thereby improving the cell's utilization of theincident light and increasing the electrical current it can generate.Thus, the insulating layer is often denoted as an anti-reflectivecoating (ARC). The thickness of the layer preferably is chosen tomaximize the anti-reflective property in accordance with the layermaterial's composition and refractive index. In one approach, thedeposition processing conditions are adjusted to vary the stoichiometryof the layer, thereby altering properties such as the refractive indexto a desired value. For a silicon nitride layer with a refractive indexof about 1.9 to 2.0, a thickness of about 700 to 900 Å (70 to 90 nm) issuitable.

The insulating layer may be deposited on the substrate by methods knownin the microelectronics art, such as any form of chemical vapordeposition (CVD) including plasma-enhanced CVD (PECVD) and thermal CVD,thermal oxidation, or sputtering. In another embodiment, the substrateis coated with a liquid material that under thermal treatment decomposesor reacts with the substrate to form the insulating layer. In stillanother embodiment, the substrate is thermally treated in the presenceof an oxygen- or nitrogen-containing atmosphere to form an insulatinglayer. Alternatively, no insulating layer is specifically applied to thesubstrate, but a naturally forming substance, such as silicon oxide on asilicon wafer, may function as an insulating layer.

The present method optionally includes the step of forming theinsulating layer on the semiconductor substrate prior to the applicationof the paste composition.

In some implementations of the present process, the paste composition isapplied over any insulating layer present on the substrate, whetherspecifically applied or naturally occurring. The paste's fusiblematerial and any additive present may act in concert to combine with,dissolve, or otherwise penetrate some or all of the thickness of anyinsulating layer material during firing. Preferably, good electricalcontact between the paste composition and the underlying semiconductorsubstrate is thereby established. Ideally, the firing results in asecure attachment of the conductive metal structure to the substrate,with a metallurgical bond being formed over substantially all the areaof the substrate covered by the conductive element. In an embodiment,the conductive metal is separated from the silicon by a nanometer-scaleinterfacial film layer (typically of order 5 nm or less) through whichthe photoelectrons tunnel. In another embodiment, contact is madebetween the conductive metal and the silicon by a combination of directmetal-to-silicon contact and tunneling through thin interfacial filmlayers.

Firing also promotes the formation of both good electrical conductivityin the conductive element itself and a low-resistance connection to thesubstrate, e.g., by sintering the conductive metal particles and etchingthrough the insulating layer. While some embodiments may function withelectrical contact that is limited to conductive domains dispersed overthe printed area, it is preferred that the contact be uniform oversubstantially the entire printed area.

Structures

An embodiment of the present invention relates to a structure comprisinga substrate and a conductive electrode, which may be formed by theprocess described above.

Semiconductor Device Manufacture

The structures described herein may be useful in the manufacture ofsemiconductor devices, including photovoltaic devices. An embodiment ofthe invention relates to a semiconductor device containing one or morestructures described herein. Another embodiment relates to aphotovoltaic device containing one or more structures described herein.Still further, there is provided a photovoltaic cell containing one ormore structures described herein and a solar panel containing one ormore of these structures.

In various embodiments, the present paste composition is useful inconstructing photovoltaic devices on silicon wafers having a variety ofdoping profiles. For example, these devices are frequently constructedon ˜200 μm p-type wafers having a ˜0.4 μm layer of n-type Si on thewafer's front surface to serve as the emitter, Cells can be constructedusing the present paste composition on wafers with a range ofconcentrations and profiles of the dopant, including cells commonlyregarded as having a highly or heavily doped emitter (HDE) wherein the Pdopant at the surface [P_(surface)] ranges from 9 to 15×10²⁰ atoms/cm³.The active [P_(surface)] in these wafers typically ranges from 3 to4×10²⁰ atoms/cm³. The paste composition is also useful with lightly orlow doped emitters (LDE) having [P_(surface)]<1×10²⁰ atoms/cm³.

In another aspect, the present invention relates to a device, such as anelectrical, electronic, semiconductor, photodiode, or photovoltaicdevice. Various embodiments of the device include a junction-bearingsemiconductor substrate and an insulating layer, such as a siliconnitride layer, present on a first major surface of the substrate.

One possible sequence of steps implementing the present process formanufacture of a photovoltaic cell device is depicted by FIGS. 1A-1F.

FIG. 1A shows a p-type substrate 10, which may be single-crystal,multi-crystalline, or polycrystalline silicon. For example, substrate 10may be obtained by slicing a thin layer from an ingot that has beenformed from a pulling or casting process. Surface damage andcontamination (from slicing with a wire saw, for example) may be removedby etching away about 10 to 20 μm of the substrate surface using anaqueous alkali solution such as aqueous potassium hydroxide or aqueoussodium hydroxide, or using a mixture of hydrofluoric acid and nitricacid. In addition, the substrate may be washed with a mixture ofhydrochloric acid and optional hydrogen peroxide to remove heavy metalssuch as iron adhering to the substrate surface. Substrate 10 may have afirst major surface 12 that is textured to reduce light reflection.Texturing may be produced by etching a major surface with an aqueousalkali solution such as aqueous potassium hydroxide or aqueous sodiumhydroxide. Substrate 10 may also be formed from a silicon ribbon.

In FIG. 1B, an n-type diffusion layer 20 is formed to create a p-njunction with p-type material below. The n-type diffusion layer 20 canbe formed by any suitable doping process, such as thermal diffusion ofphosphorus (P) provided from phosphorus oxychloride (POCl₃) or ionimplantation. In the absence of any particular modifications, the n-typediffusion layer 20 is formed over the entire surface of the siliconp-type substrate. The depth of the diffusion layer can be varied bycontrolling the diffusion temperature and time, and is generally formedin a thickness range of about 0.3 to 0.5 μm. The n-type diffusion layermay have a sheet resistivity from several tens of ohms per square up toabout 120 ohms per square.

After protecting one surface of the n-type diffusion layer 20 with aresist or the like, the n-type diffusion layer 20 is removed from mostsurfaces by etching so that it remains only on the first major surface12 of substrate 10, as shown in FIG. 1C. The resist is then removedusing an organic solvent or the like.

Next, as shown in FIG. 1D, an insulating layer 30, which also functionsas an anti-reflective coating, is formed on the n-type diffusion layer20. The insulating layer is commonly silicon nitride, but can also be alayer of another material, such as SiN_(x):H (i.e., the insulating layercomprises hydrogen for passivation during subsequent firing processing),titanium oxide, silicon oxide, mixed silicon oxide/titanium oxide, oraluminum oxide. The insulating layer can be in the form of a singlelayer or multiple layers of the same or different materials.

Next, electrodes are formed on both major surfaces 12 and 14 of thesubstrate. As shown in FIG. 1E, a paste composition 500 of thisinvention is screen printed on the insulating layer 30 of the firstmajor surface 12 and then dried. For a photovoltaic cell, pastecomposition 500 is typically applied in a predetermined pattern ofconductive lines extending from one or more bus bars that occupy apredetermined portion of the surface. In addition, aluminum paste 60 andback-side silver paste 70 are screen printed onto the back side (thesecond major surface 14 of the substrate) and successively dried. Thescreen printing operations may be carried out in any order. For the sakeof production efficiency, all these pastes are typically processed byco-firing them at a temperature in the range of about 700° C. to about975° C. for a period of from several seconds to several tens of minutesin air or an oxygen-containing atmosphere. An infrared-heated beltfurnace is conveniently used for high throughput.

As shown in FIG. 1F, the firing causes the depicted paste composition500 on the front side to sinter and penetrate through the insulatinglayer 30, thereby achieving electrical contact with the n-type diffusionlayer 20, a condition known as “fire through.” This fired-through state,i.e., the extent to which the paste reacts with and passes through theinsulating layer 30, depends on the quality and thickness of theinsulating layer 30, the composition of the paste, and on the firingconditions. A high-quality fired-through state is believed to be animportant factor in obtaining high conversion efficiency in aphotovoltaic cell. Firing thus converts paste 500 into electrode 501, asshown in FIG. 1F.

The firing further causes aluminum to diffuse from the back-sidealuminum paste into the silicon substrate, thereby forming a p+ layer40, containing a high concentration of aluminum dopant. This layer isgenerally called the back surface field (BSF) layer, and helps toimprove the energy conversion efficiency of the solar cell. Firingconverts the dried aluminum paste 60 to an aluminum back electrode 61.The back-side silver paste 70 is fired at the same time, becoming asilver or silver/aluminum back electrode 71. During firing, the boundarybetween the back-side aluminum and the back-side silver assumes thestate of an alloy, thereby achieving electrical connection. Most areasof the back electrode are occupied by the aluminum electrode, owing inpart to the need to form a p+ layer 40. Since there is no need forincoming light to penetrate the back side, substantially the entiresurface may be covered. At the same time, because soldering to analuminum electrode is unfeasible, a silver or silver/aluminum backelectrode is formed on limited areas of the back side as an electrode topermit soldered attachment of interconnecting copper ribbons or thelike.

While the present invention is not limited by any particular theory ofoperation, it is believed that, upon firing, the boron lithium telluriumoxide material, with any additive component present acting in concert,promotes rapid etching of the insulating layer conventionally used onthe front side of a photovoltaic cell. Efficient etching in turn permitsthe formation of a low resistance, front-side electrical contact betweenthe metal(s) of the composition and the underlying substrate.

It will be understood that the present paste composition and process mayalso be used to form electrodes, including a front-side electrode, of aphotovoltaic cell in which the p- and n-type layers are reversed fromthe construction shown in FIGS. 1A-1F, so that the substrate is n-typeand a p-type material is formed on the front side.

In yet another embodiment, this invention provides a semiconductordevice that comprises a semiconductor substrate having a first majorsurface; an insulating layer optionally present on the first majorsurface of the substrate; and, disposed on the first major surface, aconductive electrode pattern having a preselected configuration andformed by firing a paste composition as described above.

A semiconductor device fabricated as described above may be incorporatedinto a photovoltaic cell. In another embodiment, this invention thusprovides a photovoltaic cell array that includes a plurality of thesemiconductor devices as described, and made as described, herein.

EXAMPLES

The operation and effects of certain embodiments of the presentinvention may be more fully appreciated from a series of examples(Examples 1-4, represented by Samples 1-29) described below. Theembodiments on which these examples are based are representative only,and the selection of those embodiments to illustrate aspects of theinvention does not indicate that materials, components, reactants,conditions, techniques and/or configurations not described in theexamples are not suitable for use herein, or that subject matter notdescribed in the examples is excluded from the scope of the appendedclaims and equivalents thereof.

Example 1 Paste Preparation

In accordance with the present disclosure, a series of boron lithiumtellurium oxide materials was prepared. The compositions set forth inTable I were formulated by combining requisite amounts of the oxides orcarbonates of B, Te, Li, Na, Cr, Ca, and Zn. The amount of each oxide orcarbonate was selected to provide in the combined boron lithiumtellurium oxide the cation percentages listed in Table I.

The various ingredients for each composition were intimately mixed bymelting them in a covered Pt crucible that was heated in air from roomtemperature to 1000° C. over a period of 1 hour, and held at therespective temperature for 30 minutes. Each melt was separately pouredonto the flat surface of a cylindrically-shaped stainless steel block (8cm high, 10 cm in diameter). The cooled buttons were pulverized to a−100 mesh coarse powder.

Then the coarse powder was ball milled in a polyethylene container withzirconia media and a suitable liquid, such as water, isopropyl alcohol,or water containing 0.5 wt. % TRITON™ X-100 octylphenol ethoxylatesurfactant (available from Dow Chemical Company, Midland, Mich.) untilthe d₅₀ was in the range of 0.5 to 2 μm.

TABLE I Boron Lithium Tellurium Oxide Material Compositions cationcation cation cation cation cation cation Sample % % % % % % % # B Li TeNa Ca Cr Zn 1 35.01 19.99 45.00 2 35.01 19.99 45.00 3 35.01 19.99 45.004 34.99 12.00 44.96 4.00 4.06 5 20.00 30.00 50.00 6 20.00 15.01 64.99 730.00 15.00 55.00 8 30.00 22.50 47.50 9 20.00 22.50 57.50 10 20.00 22.5057.50 11 40.00 22.51 37.50 12 40.00 22.50 37.50 13 40.09 14.99 44.92 1430.02 29.99 39.99 15 40.00 30.01 29.99 16 30.02 29.99 39.99 17 30.0026.25 43.75 18 35.00 26.25 38.74 19 29.40 29.40 39.20 2.00 20 28.8028.80 38.40 4.00 21 27.59 27.61 36.80 8.00 22 30.00 24.00 44.00 2.00 2330.00 22.00 44.00 4.00 24 30.00 18.00 44.00 8.00

In accordance with an aspect of the invention, the boron lithiumtellurium oxide materials of Samples 1 to 24 were formulated in pastecompositions suitable for screen printing. The pastes, before adjustingtheir viscosities with additional solvent, consisted of approximately9.7 wt. % vehicle and 1.5 or 2 wt. % boron lithium tellurium oxidematerial, with the remainder being silver powder.

The organic vehicle was prepared as a master batch using a planetary,centrifugal Thinky mixer (available from Thinky USA, Inc., Laguna Hills,Calif.) to mix the ingredients listed in Table II below, withpercentages given by weight.

TABLE II Organic Vehicle Composition Ingredient wt. % 11% ethylcellulose (50-52% ethoxyl) 13.98% dissolved in TEXANOL ™ solvent 8%ethyl cellulose (48-50% ethoxyl) 5.38% dissolved in TEXANOL ™ solventtallowpropylenediamine dioleate 10.75% pentaerythritol ester ofhydrogenated 26.88% rosin Hydrogenated castor oil derivative 5.38%Dibasic ester 37.63%

For each paste, a suitable small portion of TEXANOL™ was added afterthree-roll milling to adjust the final viscosity to a level permittingthe composition to be screen printed onto a substrate. Typically, aviscosity of about 300 Pa-s was found to yield good screen printingresults, but some variation, for example ±50 Pa-s or more, would beacceptable, depending on the precise printing parameters.

Silver powder, represented by the manufacturer as having a predominantlyspherical shape, and having a particle size distribution with a d₅₀ ofabout 2.3 μm (as measured in an isopropyl alcohol dispersion using aHoriba LA-910 analyzer), was combined with the milled frit in a glassjar and tumble mixed for 15 minutes. The inorganic mixture was thenadded by thirds to a Thinky jar containing the organic ingredients andThinky-mixed for 1 minute at 2000 RPM after each addition, whereby theingredients were well dispersed in the organic vehicle. After the finaladdition, the paste was cooled and the viscosity was adjusted to betweenabout 300 and 400 Pa-s by adding solvent and Thinky mixing for 1 minuteat 2000 RPM. The paste was then milled on a three-roll mill (CharlesRoss and Son, Hauppauge, N.Y.) with a 25 μm gap for 3 passes at zeropressure and 3 passes at 100 psi (689 kPa).

Each paste composition was allowed to sit for at least 16 hours afterroll milling, and then its viscosity was adjusted to ˜300 Pa-s withadditional solvent to render it suitable for screen printing. Theviscometer was a Brookfield viscometer (Brookfield Inc., Middleboro,Mass.) with a #14 spindle and a #6 cup. Viscosity values were takenafter 3 minutes at 10 RPM.

Example 2 Fabrication and Testing of Photovoltaic Cells

Cell Fabrication

Photovoltaic cells were fabricated in accordance with an aspect of theinvention using most of the paste compositions of Example 1 to form thefront-side electrodes of the cells. The compositions used and the amountof frit (in wt. % based on the total paste composition) in each islisted in Table III below.

Conventional Gintech mono-crystalline HDE wafers (available from GintechEnergy Corporation, Jhunan Township, Taiwan) having a thickness of ˜200μm and a sheet resistivity of ˜65 ohms per square were used forfabrication and electrical testing. For convenience, the experimentswere carried out using 28 mm×28 mm “cut down” wafers prepared by dicing156 mm×156 mm starting wafers using a diamond wafering saw. The testwafers were screen printed using an AMI-Presco (AMI, North Branch, N.J.)MSP-485 screen printer, first to form a full ground plane back-sideconductor using a conventional Al-containing paste, SOLAMET® PV381(available from DuPont, Wilmington, Del.), and thereafter to form a busbar and eleven conductor lines at a 0.254 cm pitch on the front surfaceusing the various exemplary paste compositions herein. After printingand drying, cells were fired in a BTU rapid thermal processing,multi-zone belt furnace (BTU International, North Billerica, Mass.).Twenty five cells were printed using each paste; 5 cells were fired ateach set point temperature in a 5-temperature ladder ranging from 880 to940° C. After firing, the median conductor line width was about 110 μmand the mean line height was about 15 μm. The bus bar was about 1.25 mmwide. The median line resistivity was about 3.0 μΩ-cm. Performance of“cut-down” 28 mm×28 mm cells is known to be impacted by edge effectswhich reduce the overall photovoltaic cell efficiency by ˜5% from whatwould be obtained with full-size wafers.

Electrical Testing

Electrical properties of photovoltaic cells as thus fabricated using thepaste compositions of Samples 1 to 22 were measured at 25±1.0° C. usingan ST-1000 IV tester (Telecom STV Co., Moscow, Russia). The Xe arc lampin the IV tester simulated sunlight with a known intensity andirradiated the front surface of the cell. The tester used a four contactmethod to measure current (I) and voltage (V) at approximately 400 loadresistance settings to determine the cell's I-V curve. Efficiency, fillfactor (FF), and series resistance (R_(a)) were obtained from the I-Vcurve for each cell. R_(a) is defined in a conventional manner as thenegative of the reciprocal of the local slope of the IV curve near theopen circuit voltage. As recognized by a person of ordinary skill, R_(a)is conveniently determined and a close approximation for R_(s), the trueseries resistance of the cell. For each composition, an optimum firingtemperature was identified as the temperature that resulted in thehighest median efficiency, based on the 5-cell test group for eachcomposition and temperature. Electrical results for the cell groupsfired at the respective optimal firing temperature are depicted in TableIII below. Of course, this testing protocol is exemplary, and otherequipment and procedures for testing efficiencies will be recognized byone of ordinary skill in the art.

TABLE III Electrical Properties of Mono-crystalline Photovoltaic CellsSample wt. % frit Eff. FF Ra # in paste (%) (%) (ohms) 1 2.00 16.99 75.90.1748 2 1.50 17.08 76.6 0.1771 3 1.50 17.34 77.5 0.1604 4 1.50 9.5243.6 0.9829 5 1.50 17.23 77.4 0.1546 6 1.50 16.21 71.6 0.2051 7 1.5015.79 70.3 0.2889 8 1.50 17.05 75.8 0.1634 9 1.50 16.55 74.3 0.1687 101.50 16.93 76.2 0.1622 11 1.50 16.73 74.9 0.1776 12 1.50 16.95 76.90.1640 13 1.50 12.17 54.9 0.5627 14 1.50 17.18 77.9 0.1522 15 1.50 17.1477.0 0.1561 16 1.50 17.29 77.8 0.1490 19 1.50 17.25 78.1 0.1511 20 1.5017.22 77.7 0.1488 21 1.50 17.24 78.0 0.1479 22 1.50 16.46 74.4 0.1777

Example 3 Paste Preparation

In accordance with the present disclosure, another series of boronlithium tellurium oxide paste compositions was prepared, as set forth inTable IV. The same experimental procedures used to prepare the pastecompositions of Example 1 were again used.

TABLE IV Boron Lithium Tellurium Oxide Material Compositions Samplecation % cation % cation % cation % cation % # B Li Te Ti Zn 25 22 43 226.5 6.5 26 32.25 10.75 44 6.5 6.5 27 26.62 21.5 38.88 6.5 6.5 28 30.9910.75 45.26 6.5 6.5 29 21.5 21.5 44 6.5 6.5

Example 4 Fabrication and Testing of Photovoltaic Cells

Photovoltaic cells were fabricated with front-side electrodes made usingthe paste compositions (Samples 25-29) of Example 3, as set forth inTable IV. The same preparation and characterization techniques used forthe cells of Example 2 were again applied. Experiments were againcarried out using 28 mm×28 mm “cut down” wafers. Results of theelectrical testing of Examples 25-29 are shown in Table V.

TABLE V Electrical Properties of Multi-crystalline Photovoltaic CellsSample wt. % frit Eff. FF Ra # in paste (%) (%) (ohms) 25 2 15.58 72.10.1758 26 2 17.05 76.2 0.1640 27 2 16.54 74.9 0.1782 28 2 16.77 74.40.2056 29 2 16.72 74.6 0.1861

The results set forth in Table III and V demonstrate that the presentpaste composition is useful in fabricating solar cells.

Having thus described the invention in rather full detail, it will beunderstood that this detail need not be strictly adhered to but thatfurther changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the invention as defined bythe subjoined claims.

Where a range of numerical values is recited or established herein, therange includes the endpoints thereof and all the individual integers andfractions within the range, and also includes each of the narrowerranges therein formed by all the various possible combinations of thoseendpoints and internal integers and fractions to form subgroups of thelarger group of values within the stated range to the same extent as ifeach of those narrower ranges was explicitly recited. Where a range ofnumerical values is stated herein as being greater than a stated value,the range is nevertheless finite and is bounded on its upper end by avalue that is operable within the context of the invention as describedherein. Where a range of numerical values is stated herein as being lessthan a stated value, the range is nevertheless bounded on its lower endby a non-zero value.

In this specification, unless explicitly stated otherwise or indicatedto the contrary by the context of usage, where an embodiment of thesubject matter hereof is stated or described as comprising, including,containing, having, being composed of, or being constituted by or ofcertain features or elements, one or more features or elements inaddition to those explicitly stated or described may be present in theembodiment. An alternative embodiment of the subject matter hereof,however, may be stated or described as consisting essentially of certainfeatures or elements, in which embodiment features or elements thatwould materially alter the principle of operation or the distinguishingcharacteristics of the embodiment are not present therein. A furtheralternative embodiment of the subject matter hereof may be stated ordescribed as consisting of certain features or elements, in whichembodiment, or in insubstantial variations thereof, only the features orelements specifically stated or described are present. Additionally, theterm “comprising” is intended to include examples encompassed by theterms “consisting essentially of” and “consisting of.” Similarly, theterm “consisting essentially of” is intended to include examplesencompassed by the term “consisting of.”

When an amount, concentration, or other value or parameter is given aseither a range, preferred range, or a list of upper preferable valuesand lower preferable values, this is to be understood as specificallydisclosing all ranges formed from any pair of any upper range limit orpreferred value and any lower range limit or preferred value, regardlessof whether ranges are separately disclosed. Where a range of numericalvalues is recited herein, unless otherwise stated, the range is intendedto include the endpoints thereof, and all integers and fractions withinthe range. It is not intended that the scope of the invention be limitedto the specific values recited when defining a range.

In this specification, unless explicitly stated otherwise or indicatedto the contrary by the context of usage,

(a) amounts, sizes, ranges, formulations, parameters, and otherquantities and characteristics recited herein, particularly whenmodified by the term “about”, may but need not be exact, and may also beapproximate and/or larger or smaller (as desired) than stated,reflecting tolerances, conversion factors, rounding off, measurementerror, and the like, as well as the inclusion within a stated value ofthose values outside it that have, within the context of this invention,functional and/or operable equivalence to the stated value; and

(b) all numerical quantities of parts, percentage, or ratio are given asparts, percentage, or ratio by weight; the stated parts, percentage, orratio by weight may or may not add up to 100.

What is claimed is:
 1. A paste composition comprising: an inorganicsolids portion comprising: a source of electrically conductive metal anda boron lithium tellurium oxide containing boron, lithium and telluriumcations that together comprise at least 80% of the cations present inthe boron lithium tellerium oxide, and wherein the minimum content ofboron, lithium and tellurium cations is at least 15, 10 and 20 cation%,respectively; and an organic vehicle in which the source of electricallyconductive metal and the boron lithium tellurium oxide are dispersed. 2.The paste composition of claim 1, wherein the boron lithium telluriumoxide comprises 15 to 40 cation % B, 10 to 45 cation % Li, and 20 to 65cation % Te.
 3. The paste composition of claim 1, wherein the boron,lithium, and tellurium cations together comprise 80 to 100 cation % ofthe boron lithium tellurium oxide.
 4. The paste composition of claim 3,wherein the boron lithium tellurium oxide comprises: 15 to 40 cation %of B; 10 to 45 cation % of Li; 20 to 65 cation % of Te; 0 to 20 cation %of Ca; 0 to 20 cation % of Sr; 0 to 20 cation % of Ba; 0 to 15 cation %of Mg; 0 to 15 cation % of Na; 0 to 17 cation % of Si; 0 to 10 cation %of Cr; 0 to 12 cation % of Zn; and 0 to 10 cation % of Ti, plusincidental impurities.
 5. The paste composition of claim 1, wherein theboron lithium tellurium oxide further comprises up to 15 cation % of atleast one oxide selected from the group consisting of oxides of one ormore of Al, Na, K, Rb, Cs, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb,Si, Mo, Hf, W, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, Bi, P, Y, La and theother lanthanide elements, and mixtures thereof.
 6. The pastecomposition of claim 5, wherein the boron lithium tellurium oxidecomprises up to 15 cation % of at least one oxide selected from thegroup consisting of oxides of Ti, Cr, Zn, Fe, Mn, Na, and K.
 7. Thepaste composition of claim 1, wherein the boron lithium tellurium oxidefurther comprises up to 4.5 cation % of one or more of the alkalineearth metals Mg, Ca, Sr, and Ba.
 8. The paste composition of claim 7,wherein the boron lithium tellurium oxide comprises 20 to 29.5 cation %Te.
 9. The paste composition of claim 1, wherein the boron lithiumtellurium oxide comprises 20 to 24.5 cation % Te.
 10. The pastecomposition of claim 1, wherein the boron lithium tellurium oxidefurther comprises up to 5 cation % of Pb.
 11. The paste composition ofclaim 1, wherein up to 10 anion percent of the oxygen anions of theboron lithium tellurium oxide are replaced by halogen anions.
 12. Thepaste composition of claim 1, comprising 1 to 10 weight % of the boronlithium tellurium oxide.
 13. The paste composition of claim 1, whereinthe source of the electrically conductive metal is an electricallyconductive metal powder.
 14. The paste composition of claim 1, whereinthe electrically conductive metal comprises Ag.
 15. The pastecomposition of claim 14, wherein the Ag comprises 75 to 99 wt. % of theinorganic solids portion in the composition.
 16. The paste compositionof claim 1, wherein the paste composition is lead-free.
 17. The pastecomposition of claim 1, further comprising at least one discrete oxideadditive selected from the group consisting of an oxide of Al, Li, Na,K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb,Si, Mo, Hf, W, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, Bi, P, Y, La and theother lanthanide elements, and mixtures thereof, and a substance whichforms such an oxide upon firing.
 18. A process for forming anelectrically conductive structure on a substrate, the processcomprising: (a) providing a substrate having a first major surface andan insulating layer thereon that comprises aluminum oxide, titaniumoxide, silicon nitride, SiN_(x):H, silicon oxide, or siliconoxide/titanium oxide; (b) applying a paste composition onto apreselected portion of the insulating layer on the first major surface,wherein the paste composition comprises: i) a source of electricallyconductive metal, ii) a boron lithium tellurium oxide containing boron,lithium and tellurium cations that together comprise at least 80% of thecations present in the boron lithium tellurium oxide, and wherein theminimum content of boron, lithium and tellurium cations is at least 15,10 and 20 cation%, respectively; and iii) an organic vehicle in whichthe source of electrically conductive metal and the boron lithiumtellurium oxide are dispersed; and (c) firing the substrate and pastecomposition thereon, wherein the insulating layer is penetrated and theelectrically conductive metal is sintered during the firing to form theelectrically conductive structure and provide electrical contact betweenthe electrically conductive metal and the substrate.
 19. An articlecomprising a substrate and an electrically conductive structure thereon,the article having been formed by the process of claim
 18. 20. Thearticle of claim 19, wherein the article comprises a photovoltaic cell.